In the century that followed the union of England and Scotland, Ireland gained effective legislative independence from Great Britain through the Constitution of 1782. However, access to institutional power in Ireland was restricted to a small minority, the so-called Protestant Ascendancy and frustration at the lack of reform eventually led to a rebellion in 1798, involving a French invasion of Ireland and seeking complete independence from Great Britain. The rebellion was crushed with much bloodshed, and the subsequent drive for union between Great Britain and Ireland that passed in 1800 was motivated at least in part by the belief that the rebellion was caused as much by loyalist brutality as by the United Irishmen.

Each Act had to be passed in the Parliament of Great Britain and the Parliament of Ireland.

After centuries of subordination to the English, and later, British Parliaments, the Parliament of Ireland gained a large measure of independence by the Constitution of 1782. Many members of the Irish Parliament jealously guarded its autonomy (notably Henry Grattan) and a motion for union was rejected in 1799. However, a concerted campaign by the British government overcame the reluctance of the Irish Parliament.

Only Anglicans were permitted to become members of the Parliament of Ireland, though the great majority of the Irish population were Roman Catholic, with many Presbyterians in Ulster. In 1793 Roman Catholics regained the right to vote if they owned or rented property worth £2 p.a. The Catholic hierarchy was strongly in favour of union, hoping for rapid emancipation – the right to sit as MPs – which was however delayed until 1829.

From the perspective of Great Britain, the union was required because of the uncertainty that followed the Irish Rebellion of 1798 and the French Revolution of 1789, which inspired the rebels; if Ireland adopted Catholic Emancipation, willingly or not, a Roman Catholic parliament could break away from Britain and ally with the French, while the same measure within a united kingdom would exclude that possibility. Also the Irish and British parliaments, when creating a regency during King George III's "madness", gave the Prince Regent different powers. These considerations led Great Britain to decide to merge the two kingdoms and their parliaments.

The final passage of the Act in the Irish Parliament was achieved with substantial majorities, achieved in part according to contemporary documents through bribery, namely the awarding of peerages and honours to critics to get their votes.[6] Whereas the first attempt had been defeated in the Irish House of Commons by 109 votes against to 104 for, the second vote in 1800 produced a result of 158 to 115.[6]

The House of Commons was to include the pre-union representation from Great Britain and 100 members from Ireland: two members from each of the 32 counties and from the two largest boroughs, and one from each of the next 31 boroughs and from Dublin University. The other 84 Irish parliamentary boroughs were disfranchised; all were pocket boroughs, whose patrons received £15,000 compensation for the loss of what was considered their property.

Article V united the established Church of England and Church of Ireland into "one Protestant Episcopal Church, to be called, The United Church of England and Ireland"; but also confirmed the independence of the Church of Scotland.

Article VI created a customs union, with the exception that customs duties on certain British and Irish goods passing between the two countries would remain for 10 years (a consequence of having trade depressed by the ongoing war with revolutionary France).

Article VII stated that Ireland would have to contribute two-seventeenths towards the expenditure of the United Kingdom. The figure was a ratio of Irish to British foreign trade.

Article VIII formalised the legal and judicial aspects of the Union.

Part of the attraction of the Union for many Irish Catholics was the promise of Catholic Emancipation, allowing Roman Catholic MPs, who had not been allowed in the Irish Parliament. This was however blocked by King George III who argued that emancipating Roman Catholics would breach his Coronation Oath, and was not realised until 1829.

In the first Parliament of the United Kingdom of Great Britain and Ireland, the members of the House of Commons were not elected afresh. By royal proclamation authorized by the Act, all the members of the last House of Commons from Great Britain took seats in the new House, and from Ireland 100 members were chosen from the last Irish House of Commons; both members from each of the 32 counties and from the two largest boroughs, and one each (chosen by lot) from the next 31 boroughs and from Dublin University.